1. Field of the Invention
The invention relates in general terms to a unit, in particular a melting and/or refining unit and/or a distributor system and/or a channel system, for conductively heatable melts, and specifically to a unit for conductively heatable glass melts.
2. Description of Related Art
The production of glasses involves the formation of glass melts to which thermal energy is supplied. This applies to the process of melting down glass or glass charge cullet, as well as subsequent process steps, such as for example refining or homogenization. The thermal energy which this requires can particularly efficiently be released directly in the melt by means of the Joule effect.
Therefore, melting installations, in particular for glassmaking, often use electrodes which are immersed in the liquid melt. Electric current is introduced into the melt via the electrodes. The electrode unit in this case comprises the electrode body and an electrode holder which carries the electrode body. The electrode holder and the electrode body are generally fixedly connected to one another, for example by screw connection or welding. For its part, the electrode holder is fixedly connected to the external surroundings.
Electrodes are used immersed in the melt both in the base and in the sides or from above. The form of electrode which is most frequently used is the stick shape, since this is very easy to push in further. Moreover, the stick-shaped electrode offers the advantage that it is possible to exchange the entire arrangement made of electrode holder and electrode body even when the installation is running. In the case of plate electrodes, by contrast, it is not possible to either exchange the electrode or push it in further during operation.
Above a certain temperature, glasses become electrically conductive, so that at voltages up to approximately 1000 V, sufficiently high currents can usually flow to heat the glass melt using the Joule effect. In the melt, the current is substantially transported by the ions of different mobility. Standard frequencies for the conductive heating of melts are 50 or 60 Hz; frequencies of 10 kHz are used for high-quality glasses, in particular optical glasses.
FIG. 1 illustrates a typical structure which illustrates how the current is fed into the melt from outside the unit via an electrode. The figure shows an excerpt from the wall of a melting and/or refining unit having an electrode assembly. The arrangement comprises a generally water-cooled electrode holder, in which the actual electrode body is secured in a suitable way. On the outside, the electrode holder is connected to a heating circuit transformer via a cable. The electrode is introduced into the melt through the wall, which is constructed from refractory material. The distance, in the longitudinal direction of the electrode holder or electrode body, from the upper side of the electrode holder, facing the melt, to that surface of the wall which faces the melt is referred to as the setting depth. The setting depth is determined according to the particular type of glass, in particular its vitrification and crystallization properties, as well as the required process temperatures and the structure of the wall, in particular the thermal conductivity of the materials used.
It is customary to use separate electrode bricks in which there is a drilled hole for the electrode. The electrode brick itself is inserted into the wall of the melting and/or refining unit. A separate electrode brick offers the advantage that the region around the electrode can be easily observed from the outside. It is in this way possible to quickly recognize cracks and glass leakages. If the thickness of the brick is reduced by the glass melt on account of corrosion, it is easy to apply forced cooling from the outside, which efficiently cools that side of the electrode brick which faces the glass and therefore reduces the corrosion caused by the glass melt.
Details can be found in the relevant specialist literature on glass technology. The prior art is well documented, for example, in the HVG training volumes “Elektroschmelzen von Glas” [Electrical melting of glass] 1990 and “Wärmetransportprozesse bei der Herstellung und Formgebung von Glas” [Heat transfer processes in the production and shaping of glass] 2002.
The current, which is fed in from the outside from suitable matching transformers via feed lines, enters the glass melt from the electrode body in order to heat the glass melt by the Joule effect. The electrical power density ρel({right arrow over (r)}) for the entire arrangement comprising melt and wall, given the standard dimensions of the units and heating frequencies, can be calculated as:Pel({right arrow over (r)})={right arrow over (J)}({right arrow over (r)})· E({right arrow over (r)})=ρel(T)·{right arrow over (J)}2({right arrow over (r)})=σel(T)·Ē2({right arrow over (r)})  (1)where Pel({right arrow over (r)}) has the dimension
      [          W              m        3              ]    .Ē is the electric field in
      [          V      m        ]    ⁢          ⁢  and  ⁢          ⁢      J    _  is the current density in
  [      A          m      2        ]at any location {right arrow over (r)} of the arrangement observed. ρel denotes the electrical resistivity in [Ω·m] and σel denotes the electrical conductivity of the materials used in
      [          S      m        ]    .
The electrical resistivity and the electrical conductivity are temperature-dependent. In the case of the glasses and refractory materials that are customarily used, they are generally negatively temperature-dependent. This means that with increasing temperature, the electrical conductivity increases or the electrical resistivity decreases.
The electrical power density ρel introduced into the entire arrangement according to equation 1 first of all leads to an increase in temperature ΔT at any point {right arrow over (r)}. In glass melts, the heat quantity {dot over (q)}Pel generated on account of the current in a local volume element can be dissipated again from the volume element under consideration with the aid of three mechanisms.
The first mechanism is heat conduction, which is imparted by phonons. It is assigned a {dot over (q)}heat conduction. The second mechanism for dissipating heat from a specific volume element of the melt is radiation (heat flow {dot over (q)}radiation), the exchange particles of which are photons. Finally, heat can also be dissipated from the melt by means of convection flows by a heat flow {dot over (q)}convection. All three mechanisms are temperature-dependent. In general, apart from exceptions in the case of what are known as “dark” glasses, radiation is the dominant process.
After the start-up phase, a steady state is established at a defined temperature Teq, in which the heat flows involved are in equilibrium with one another. In the steady state at T=Teq, the following relationship applies:{dot over (q)}Pel={dot over (q)}radiation+{dot over (q)}convection={dot over (q)}heat conduction  (2)
After the heat quantity generated by the electric current has been dissipated from a volume element of the melt, a volume element of the material of the wall of the unit will now be considered. The refractory materials which are in contact with glass melts can be roughly divided into three groups. What are known as the HZFC (high zirconia fused cast) materials, which are cast at temperatures around 2300° C., become soft beyond 1900° C. to 2000° C. AZS (alumina zirconia silica) materials, the casting temperatures of which are 1900° C. to 2000° C., do not become soft, but rather decompose at a temperature above approximately 1800° C. ZS (zirconium silicate) materials, which are sintered in pressed form during production, decompose above temperatures of just 1700° C. In any event, high excess temperatures in the refractory material lead to destruction of the latter.
At high temperatures, many refractory materials have an electrical conductivity comparable to that of glasses. In the refractory material itself, the heat quantity which is locally released by the electric current, contrary to the situation for the melt as described above, however, can substantially only be dissipated by the mechanism of pure heat conduction.
If, in the volume element under consideration, the dissipation of heat from the refractory material is lower than the heat quantity {dot over (q)}Pel generated in this volume element, the temperature will rise. In the volume element under consideration, this is associated with an increased electrical conductivity on account of the negative temperature dependency of the electrical resistivity. At constant voltage and therefore with a constant electric field E, a rise in the electrical conductivity σel(T) in accordance with equation 1 is associated with a higher electric power density Pel({right arrow over (r)}). Accordingly, the temperature rises further. This process repeats itself until, at a relatively high temperature, a new equilibrium state is reached in the refractory material of the wall of the unit.
In a situation which can still be tolerated, all that happens is that the usual corrosion of the refractory material is accelerated on account of the higher temperature. The service life of the unit is shortened as a result. In the least favorable situation, however, the system goes out of control, leading to destruction of regions of the refractory material caused by partial melting or rapid decomposition.
In the case of refractory materials, it is not just the electrical resistivity in the new state which needs to be taken into consideration, but also the possibility that this resistivity may change during operation, for example as a result of the introduction of glass constituents, in particular caused by alkali metal diffusion. The material becomes more electrically conductive as a result, thereby increasing the risk of local instabilities in the refractory material.
If the temperatures in the wall of the unit are higher than in the melt, the melt itself functions as a cooling medium for the wall of the unit. The dissipation of heat from the wall can be improved by adapting the insulation on the outer side. The same applies to externally applied forced cooling with air.
In addition to the advantage of improving the dissipation of heat from the wall, however, the abovementioned measures also bring with them serious drawbacks. Although the risk of excess temperatures is reduced, the cooling or reduced insulation means unnecessary additional heat loss. As a result, the efficiency of the overall installation is adversely affected to a considerable extent. On account of the altered temperatures and heat flows, furthermore, flow phenomena may occur in the glass melt, having an adverse effect on the quality of the process.
The problem mentioned above is caused in particular by the material forming the electrode brick. This material needs to be able to withstand temperature changes, since water-cooled electrode holders are generally used. In the case of electrodes which can be pushed in further, the water supply to the electrode holder is interrupted a number of times during a tank campaign, with the result that the electrode brick is exposed to very considerable temperature gradients within a short period of time.
During the heating of the installation and/or on account of the abovementioned high temperature gradients, cracks may form in the electrode brick. These cracks generally extend radially outward from the electrode bore in the brick. Glass can penetrate into cracks of this type, thereby accelerating the corrosion of the electrode brick.
Therefore, a brick material which is as insensitive as possible to temperature changes should be selected. However, other properties of the brick, such as its resistance to corrosion from the glass melt and the electrical resistivity, should also be taken into consideration.
The ideal refractory material for an electrode brick should satisfy the following criteria. It should have only a low susceptibility to corrosion on contact with the melts used and also a high resistance to temperature changes. Moreover, the electrical resistivity in the technologically relevant temperature range should be significantly higher than that of the corresponding melt. Moreover, the material should ideally have a high thermal conductivity, in order to allow even small quantities of electrical energy released to be efficiently dissipated in the refractory material.
Many refractory materials, which have a high electrical resistivity compared to glass melts, however, have a poor ability to withstand temperature changes, as for example in the case of the zirconium silicate material ZS 1300. The higher the temperature gradient and its spatial profile in the refractory material, the more critical this problem becomes. If refractory materials with a high electrical resistivity compared to the melt are ruled out on account of their poor resistance to corrosion from glass melts, it is necessary to employ materials which are more resistant to the glass melts but under certain circumstances have a higher electrical conductivity.
On account of the required resistance to corrosion from the melt, the ability to withstand temperature changes, the availability and price, there is a very limited choice of suitable refractory materials.
To avoid the risk of excess temperatures, therefore, adapting the electrical heating by suitably selecting the heating circuit geometries, the phase positions and the electrode positions in the melting and/or refining unit has been the only viable solution known hitherto. By way of example, the study entitled “Elektrotechnische und wärmetechnische Untersuchungen zur Auswahl von Feuerfestmaterialien für Elektroschmelzöfen zur Glasschmelze” [Electrical engineering and heat engineering tests on the selection of refractory materials for electric melting furnaces for glass melts] by H.-J. Illig et al., XI International Glass Congress, Prague 1977, anthology V, recommends that a maximum electric field strength of 4-5 V/cm should not be exceeded using typical materials for the tank. However, this imposes considerable restrictions on the way in which the installation is operated.
One possible option is, for example, a switch from bottom electrodes to what are known as top electrodes, which are immersed in the melt from above and do not have to be passed through a refractory structure.
In many cases, it would be desirable for the electrical heating circuits to be operated with a higher power and therefore higher voltages, in order, for example, to achieve higher throughputs or to optimize the flows with regard to glass quality. If all the optimization options, in particular those presented above, have already been exhausted, in many cases the diameters of the electrode sticks are increased, in order to influence the current densities and therefore the local release of energy in the immediate vicinity of the electrode. However, the electrode diameter cannot continue to be increased arbitrarily, since the size of the drilled hole in the refractory material is limited.
The underlying problem in the context outlined above resides in the spatially extremely nonuniform distribution of the introduction of heating power into the wall of the tank, which causes damage to the refractory material.